The present invention relates to a water-absorptive soft contact lens, and more particularly to a water-absorptive soft contact lens which has excellent oxygen permeability and high mechanical strength independent of water content, and furthermore, which is excellent in transparency and shape stability.
Usually, in contact lens materials, there are hard materials and soft materials. In order to obtain a contact lens which is excellent in sensation of wearing, in general, soft materials are suitably used. In the soft materials, there are water-absorptive materials which swell and soften when absorbing water and substantially non-water-absorptive materials.
In order to be able to wear a contact lens in an eye more safely for a long period of time, it is necessary that more oxygen is supplied for a cornea. Therefore, there is employed a method for substantially heightening oxygen permeability comprising heightening oxygen permeability of materials or reducing the thickness of a lens.
The above-mentioned water-absorptive material itself does not permeate oxygen, and the oxygen permeability is imparted to the material through the water absorbed in the material. For instance, as is clear from the description of "OXYGEN PERMEABILITY OF CONTACT LENS MATERIALS: A 1993 UPDATE" by Fatt and Ruben, which is described in "Journal of the British Contact Lens Association", 17 [1](1994), pages 11 to 18 (hereinafter referred to as "Journal of Fatt et al"), in general, it is known that oxygen permeability depends on water content (see a graph denoted with the mark B in FIG. 1). Accordingly, in order to heighten the oxygen permeability of a contact lens, a material having a high water content is necessitated. However, in general, there arises a disadvantage such that the mechanical strength of the material becomes lower in accordance with the increase of water content of the material.
When a low water-absorptive material is used, there has been attempted the diminishing of the thickness of a lens made of the material to substantially heighten its oxygen permeability. However, the production of a thin lens which can supply sufficient oxygen for a cornea is technically difficult. Also, since thinner the lens is, the lower the shape stability of the lens becomes, there arises a problem when wearing a thin lens in an eye such that handling of the lens becomes difficult during wearing in an eye.
The present invention has been accomplished in consideration of the above prior art.
An object of the present invention is to provide a water-absorptive soft contact lens which has excellent oxygen permeability and high mechanical strength independent of water content.
Another object of the present invention is to provide a
water-absorptive soft contact lens which is excellent in transparency, flexibility and shape stability.
These and other objects of the present invention will become apparent from the description hereinafter.